This invention relates generally to an electrophotographic printing machine, and more particularly, concerns a control system for regulating the illumination intensity of the optical system as a function of the spacing between the screen and photoconductive surface.
As in all electrophotographic printing machines, the original document being reproduced is illuminated to form a light image thereof. The light image irradiates the charged portion of the photoconductive surface dissipating selectively the charge thereon to record thereon an electrostatic latent image. Heat settable particles develop the latent image. These particles are transferred to a sheet of support material, in image configuration. Thereafter, heat is applied to the particles permanently affixing them to the sheet of support material.
In the process of multi-color electrophotographic printing, a plurality of electrostatic latent images are recorded on the photoconductive member. Each latent image corresponds to a different color of the original document. Differently colored toner particles are utilized to develop the latent images. These particles are then transferred from the latent image to the sheet of support material, in superimposed registration with one another. The resultant multi-layered powder image is then permanently affixed to the sheet of support material by the application of heat thereto. In this manner, a color copy of the original document is formed.
An electrophotographic printing machine may be arranged to produce either a functional copy or a pictorial copy. A functional copy is a copy of a document wherein subtle variations of tone or color are not present, such as in a graph, chart, lines, etc. Pictorial quality copies are reproduced in electrophotographic printing machines by employing half tone imaging screens. These screens produce tone gradations by forming half-tone dots or lines of varying sizes. The dots increase in size from the highlight regions throughout the intermediate shades until they merge together in the shadow regions. Numerous patents teach the concept of screening. Exemplary of these patents are U.S. Pat. Nos. 2,598,732; 3,535,036; 3,493,381;3,776,633; and 3,809,555. It has been found that the contrast of a pictorial copy may be regulated by adjusting the spacing between the photoconductive surface and the screen. A functional copy may be produced by removing the screen from the optical light path so that the light image is not modulated. Alternatively, the spacing may be increased to an optimum distance so as to defocus the screen member. Thus, by increasing the spacing between the screen member and photoconductive member a sufficient distance, the screen is de-focused and is ineffective. Typical systems for moving the screen relative to the photoconductive surface are described in copending application Ser. No. 566,872, filed Apr. 10, 1975, now U.S. Pat. No. 3,958,877, copending application Ser. No. 567,149, filed Apr. 11, 1975, now U.S. Pat. No. 3,961,848, and copending application Ser. No. 647,289, filed Jan. 7, 1976, now U.S. Pat. No. 4,025,181. However, the spacing between the screen and photoconductive surface also appears to affect the intensity of illumination irradiating the photoconductive surface.
Accordingly, it is a primary object of the present invention to improve the optical system of an electrophotographic printing machine by automatically adjusting the intensity of illumination as a function of the spacing between the screen and photoconductive surface.